Adaptive space-time transmit diversity coding for MIMO systems

ABSTRACT

A method for transmitting a stream of data symbols in a multiple-input/multiple-output (MIMO) wireless communications system including N r  transmitting antennas. The stream of data symbols is first demultiplexed into M sub-streams, where M=N r /2. Then, space-time transmit diversity encoding is applied to each sub-stream to generate a pair of transmit signals. Power is allocated dynamically to each transmit signal of each pair of transmit signals according to a corresponding feedback signal received from a receiver of the transmit signal. The feedback signal including a ratio of magnitude sums of channel coefficients for channels used for the transmit signals.

RELATED APPLICATION

This is a continuation-in-part application of U.S. patent applicationSer. No. 10/217,919 “MIMO Systems with STTD Encoding and Dynamic PowerAllocation,” filed by Horng et al., on Aug. 13, 2002.

FIELD OF THE INVENTION

The invention relates generally to wireless communications, and moreparticularly to multiple input/multiple output wireless communicationssystems with dynamic power allocation.

BACKGROUND OF THE INVENTION

Transmit diversity is one of the key technologies used in thirdgeneration (3G) wireless communications systems. By transmitting thesignal through multiple transmit antennas to multiple receive antennasspatial diversity gains can be achieved to enhance the system capacity.Space-time transmit diversity (STTD) is an open loop technique in whichthe symbols are modulated using space-time block code, see Alamouti, “Asimple transmit diversity technique for wireless communications,” IEEEJ. Select. Areas Commun., 16:1451-1468, 1998.

The STTD scheme is adopted by 3GPP due to its simple implementation andmaximal diversity gains. Transmit adaptive array (TXAA) is a closed looptransmit diversity technique adopted by 3GPP. The mobile receiverfeedbacks the estimated optimal transmit weights to the basestation. Thebase station uses this feedback information to adjust the power level ofthe transmitted signal so that the received power at the desired mobilereceiver is maximized.

Simulation results show that the STTD is robust at higher velocities,while TXAA provides the biggest benefits at lower velocities, seeDerryberry et al., “Transmit Diversity in 3G CDMA Systems,” IEEE Comm.Magazine, vol.40, no.4, pp. 68-75, April 2002.

A mixture of open and closed loop diversity technique could be used tocombat both fast and slow fading. Recently, an adaptive STTD (ASTTD)scheme has been described, which combines STTD with adaptive transmitpower allocation in order to improve the performances of the STTDsystems, see Huawei “STTD with Adaptive Transmitted Power Allocation,”3GPP TSG-R WG1 document, TSG-R1#26 R1-02-0711, Gyeongju, Korea May13-16, 2002.

SUMMARY OF THE INVENTION

A method for transmitting a stream of data symbols in amultiple-input/multiple-output (MIMO) wireless communications systemincluding N_(r) transmitting antennas.

The stream of data symbols is first demultiplexed into M sub-streams,where M=N_(r)/2. Then, space-time transmit diversity encoding is appliedto each sub-stream to generate a pair of transmit signals.

Power is allocated dynamically to each one of the pairs of transmittedsignal according to a corresponding feedback signal received from areceiver of the transmit signal.

The feedback signal including a ratio of magnitude sums of channelcoefficients for channels used for the transmitted signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a multiple input/multiple output wirelesscommunications systems that uses the invention; and

FIG. 2 is a block diagram of a receiver of the system of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a MIMO system 100 that can use the invention. From a singleinput signal 101, an STTD encoder 110 produces multiple output signals102 using demultiplexing 103. The power of each of the output signals isweighted 120 by a power amplifier before sent to multiple transmitterantennas 120 through channels having responses h to receiver antennas201 of a receiver 200, see FIG. 2.

The weights, w₁, and w₂, are real positive numbers which are selected tomaximize the SNR at the receiver 200 with the constraint that w₁ ²+w₂²=1. The STTD encoding 110 is a space-time block code which encodes theinput signal [X₁ X₂]^(T) 101 into output signals 102

$\begin{matrix}{\begin{bmatrix}X_{1} & {- X_{2}^{*}} \\X_{2} & X_{1}^{*}\end{bmatrix},} & (1)\end{matrix}$

where * denotes complex conjugate and each row of the matrix is assignedto the same transmit antenna 120.

FIG. 2 show the receiver 200 in greater detail. The receiver includes acombiner 220, an STTD decoder 230, and an interference cancellationstage 240.

Assume there are Nr receiver antennas 201 at the receiver 200. Areceived signal r_(i)(n) 210 at the ith receiver antenna can beexpressed as

$\begin{matrix}{\begin{matrix}\begin{bmatrix}{r_{1}(n)} \\{r_{1}^{*}( {n + 1} )} \\{r_{2}(n)} \\{r_{2}^{*}( {n + 1} )} \\\vdots \\{r_{N_{r}}(n)} \\{r_{N_{r}}^{*}( {n + 1} )}\end{bmatrix} & = & \begin{bmatrix}{w_{1}h_{11}} & {w_{2}h_{21}} \\{w_{2}h_{21}^{*}} & {{- w_{1}}h_{11}^{*}} \\{w_{1}h_{12}} & {w_{2}h_{22}} \\{w_{2}h_{22}^{*}} & {{- w_{1}}h_{12}^{*}} \\\vdots & \vdots \\{w_{1}h_{1N_{r}}} & {w_{2}h_{2N_{r}}} \\{w_{2}h_{2N_{r}}^{*}} & {{- w_{1}}h_{1N_{r}}^{*}}\end{bmatrix} & \begin{bmatrix}X_{1} \\X_{2}\end{bmatrix} & + & {\begin{bmatrix}{v_{1}(n)} \\{v_{1}^{*}( {n + 1} )} \\{v_{2}(n)} \\{v_{2}^{*}( {n + 1} )} \\\vdots \\{v_{N_{r}}(n)} \\{v_{N_{r}}^{*}( {n + 1} )}\end{bmatrix},} \\{r(n)} & \; & \overset{\sim}{H} & X & \; & {v(n)}\end{matrix}\;} & (2)\end{matrix}$

where h_(ij) are the channel coefficients of the channels between thei^(th) transmitter antennas and the j^(th) receiver antennas, and v(n)is the additive white Gaussian noise sample at time instant n, which isassumed to be independent at all receiver antenna elements.

The received signals 210 from all antennas 201 are first combined 220before passed to the STTD decoder 230. Therefore, the output {tilde over(r)} of the STTD decoder 230 corresponding to the two successivetransmitted symbol in one space-time coding block is given by

$\begin{matrix}\begin{matrix}{{\overset{\sim}{r}(n)} = {\begin{bmatrix}{\overset{\sim}{r}}_{1} \\{\overset{\sim}{r}}_{2}\end{bmatrix} = {H^{*}{r(n)}}}} \\{= {\begin{bmatrix}h_{11}^{*} & h_{21} & h_{12}^{*} & h_{22} & \cdots & h_{1N_{r}}^{*} & h_{2N_{r}} \\h_{21}^{*} & {- h_{11}} & h_{22}^{*} & {- h_{12}} & \cdots & h_{2N_{r}}^{*} & {- h_{1N_{r}}}\end{bmatrix}{r(n)}}} \\{= {{{H^{*}\overset{\sim}{H}\; X} + {H^{*}{v(n)}}} = {{\begin{bmatrix}A & B \\{- B^{*}} & A\end{bmatrix}\begin{bmatrix}X_{1} \\X_{2}\end{bmatrix}} + {\overset{\sim}{v}(n)}}}}\end{matrix} & (3) \\\begin{matrix}{{{{where}\mspace{20mu} A} = {{w_{1}{\sum\limits_{i = 1}^{N_{r}}\;{h_{1\; N_{r}}}^{2}}} + {w_{2}{\sum\limits_{i = 1}^{N_{r}}\;{h_{2\; N_{r}}}^{2}}}}},} \\{\mspace{79mu}{B = {{w_{2}{\sum\limits_{i = 1}^{N_{r}}{h_{1N_{r}}^{*}h_{2N_{r}}}}} - {w_{1}{\sum\limits_{i = 1}^{N_{r}}{h_{1N_{r}}^{*}h_{2N_{r}}}}}}}}\end{matrix} & (4) \\{and} & \; \\{{\overset{\sim}{v}(n)} = {{H^{*}{v(n)}} = \begin{bmatrix}{{\sum\limits_{i = 1}^{N_{r}}{h_{1\; i}^{*}{v_{i}(n)}}} + {\sum\limits_{i = 1}^{N_{r}}{h_{2\; i}{v_{i}^{*}( {n + 1} )}}}} \\{{\sum\limits_{i = 1}^{N_{r}}{h_{2\; i}^{*}{v_{i}(n)}}} + {\sum\limits_{i = 1}^{N_{r}}{h_{1\; i}{v_{i}^{*}( {n + 1} )}}}}\end{bmatrix}}} & (5)\end{matrix}$

To cancel the cross-interference term B in Equation (4), thecross-interference cancellation stage 240 maximizes the SNR of the STTDdecoded symbols. The output {circumflex over (X)} 250 is given by

$\begin{matrix}\begin{matrix}{\begin{bmatrix}{\hat{X}}_{1} \\{\hat{X}}_{2}\end{bmatrix} = {\begin{bmatrix}A & B \\{- B^{*}} & A\end{bmatrix}^{*}\;{\overset{\sim}{r}(n)}}} \\{= {{\begin{bmatrix}{{A}^{2} + {B}^{2}} & 0 \\0 & {{A}^{2} + {B}^{2}}\end{bmatrix}\begin{bmatrix}X_{1} \\X_{2}\end{bmatrix}} = {\begin{bmatrix}A^{*} & {- B} \\B^{*} & A\end{bmatrix}\;{\overset{\sim}{v}(n)}}}}\end{matrix} & (6)\end{matrix}$

Thus, the conditional SNR of the output signal can be obtained by

$\begin{matrix}{{{{SNR}\mspace{14mu} l_{h_{i\; j}}} = \frac{( {{A}^{2} + {B}^{2}} )E_{s}}{\sigma_{v}^{2}{\sum\limits_{j}{\sum\limits_{i}{h_{i\; j}}^{2}}}}},} & (7)\end{matrix}$

where Es is the transmitted symbol energy and σ_(v) ² is the additivewhite noise power.

Because the term A in Equation (4) contributes dominantly to the desiredsignal energy, one can maximize A instead of maximizing the SNR inEquation (7).

Thus, the optimum weight function is found by letting dA/dw₁=0 with thefixed power constraint, w₁ ²+w₂ ²=1.

$\begin{matrix}{w_{1} = {{\frac{1}{\sqrt{1 + ( \frac{\sum\limits_{j = 1}^{N_{r}}{h_{2\; j}}^{2}}{\sum\limits_{j = 1}^{N_{r}}{h_{1\; j}}^{2}} )^{2}}}\mspace{25mu} w_{2}} = {\frac{1}{\sqrt{1 + ( \frac{\sum\limits_{j = 1}^{N_{r}}{h_{1\; j}}^{2}}{\sum\limits_{j = 1}^{N_{r}}{h_{2\; j}}^{2}} )^{2}}}.}}} & (8)\end{matrix}$

Equation 8 shows that just the ratio of the magnitude sum of the channelcoefficients are sufficient for the transmitter to calculate the optimumtransmit weight. This is much simpler than the prior art closed looptechniques (TXAA), which uses the amplitude and phase information of thepropagation channels to calculate the transmit weights. It also leads toa reduced number of bits or more reliable transmission of feedbackinformation 200 in our invention.

In there are no limits on the feedback size, then an eigen-mode can beused as an alternative to calculate transmit weights. The transmitweight vector is chosen as the principal eigenvector corresponding to amaximum eigenvalue of the channel correlation matrix R, where

$\begin{matrix}{{R = {\hat{H}\;{\hat{H}}^{H}}},{{{and}\mspace{14mu}\hat{H}} = \begin{bmatrix}h_{11} & h_{12} & \cdots & h_{1N_{r}} \\h_{21} & h_{22} & \cdots & h_{2N_{r}}\end{bmatrix}},{{{for}\mspace{14mu} j} = 1},\ldots\mspace{11mu},{N_{r}.}} & (9)\end{matrix}$

The performance of the adaptive STTD can provide about a 0.8 dB SNR gainfor a BER=10⁻³, and a 1.0 dB gain for FER=10⁻¹. It should be noted thatthe invention can also be used for multiple transmit antennas and asingle receive antenna.

Although the invention has been described by way of examples ofpreferred embodiments, it is to be understood that various otheradaptations and modifications can be made within the spirit and scope ofthe invention. Therefore, it is the object of the appended claims tocover all such variations and modifications as come within the truespirit and scope of the invention.

1. A method for transmitting a stream of data symbols in amuitiple-input/multiple-output wireless communications system includingN_(r) transmitting antennas, comprising: demultiplexing the stream ofdata symbols into M sub-streams, where M=N_(r)/2; space-time transmitdiversity encoding each sub-stream into a pair of transmit signals; anddynamically allocating power to each transmit signal of each pairaccording to a corresponding feedback signal received from a receiver,the feedback signal including a ratio of magnitude sums of channelcoefficients for channels used for the transmit signal, and the power ofthe pair of transmit signals being respectively weighted by positivereal numbers w ₁ and w₂${w_{1} = {{\frac{1}{\sqrt{1 + ( \frac{\sum\limits_{j = 1}^{N_{r}}{h_{2j}}^{2}}{\sum\limits_{j = 1}^{N_{r}}{h_{1\; j}}^{2}} )^{2}}}\mspace{25mu}{and}\mspace{20mu} w_{2}} = \frac{1}{\sqrt{1 + ( \frac{\sum\limits_{j = 1}^{N_{r}}{h_{1\; j}}^{2}}{\sum\limits_{j = 1}^{N_{r}}{h_{2\; j}}^{2}} )^{2}}}}},$where h_(ij) represent the channel coefficients for a channel between ani^(th) transmitter antenna and a j^(th) receiver antenna, where j is 1,. . . , N_(r).
 2. A transmitter for a multiple-input/multiple-outputwireless communications system including N_(r), transmitting antennas,comprising: a demultiplexer configured to output M sub-streams of astream of data symbols, where M=N_(r)/2; a space-time transmit diversityencoder configured to encode each sub-stream into a pair of transmitsignals; and a power amplifier configured to dynamically allocate powerto each transmit signal of each pair according to a correspondingfeedback signal received from a receiver, the feedback signal includinga ratio of magnitude sums of channel coefficients for channels used forthe transmit signal, and the power of the pair of transmit signals beingrespectively weighted by positive real numbers w₁ and w₂,${w_{1} = {{\frac{1}{\sqrt{1 + ( \frac{\sum\limits_{j = 1}^{N_{r}}{h_{2j}}^{2}}{\sum\limits_{j = 1}^{N_{r}}{h_{1\; j}}^{2}} )^{2}}}\mspace{25mu}{and}\mspace{20mu} w_{2}} = \frac{1}{\sqrt{1 + ( \frac{\sum\limits_{j = 1}^{N_{r}}{h_{1\; j}}^{2}}{\sum\limits_{j = 1}^{N_{r}}{h_{2\; j}}^{2}} )^{2}}}}},$where h_(ij) represent the channel coefficients for a channel between ani^(th) transmitter antenna and a j^(th) receiver antenna, where j is 1,. . . , N_(r).